Missiles are conventionally steered by a laser beam transmitted from the launch site. The single circular laser beam continuously scans a field of view which is controlled to have substantially the same linear size at the missile as it flies down range. A sensor on the missile detects the beam when it scans across the missile, and electronics on the missile are able to determine where in the scanned field of view the missile is located. The missile electronics then steer the missile so as to maintain it in the center of the scanned field of view.
In order to prevent any point in the field of view from remaining unscanned, yet provide every point in the field of view with an unambiguous scan position signal, the beam needs to be wide at close range, and to become narrower as the range increases. In the prior art, this was done by providing an iris in front of the scanning device. The problem with this arrangement, however, was that as the iris closed to cause an increase in beam divergence, the beam power was greatly reduced due to the blocking of a portion of the beam. This caused significant problems in practice because the sensor on the missile needs to look through the rocket exhaust plume, and therefore needs a strong laser signal to achieve an acceptable signal-to-noise ratio.
Prior art references in this field are as follows: U.S. Pat. No. 3,516,743 to McKown et al. which describes a scanning mechanism for a laser projection system but does not address the issue of controlling the divergence of the beam; U.S. Pat. No. 3,764,192 to Wheeler which likewise describes a scanning method for a laser beam but does not address a zoom function; U.S. Pat. No. 3,954,340 to Blomqvist et al. in which a zoom lens system for target return tracking is mentioned but not described; U.S. Pat. No. 3,961,179 to Kuffer et al. which describes a scanning method but does not consider a zoom function; U.S. Pat. No. 3,989,942 to Waddoups, which describes a responder and laser beam encoder, but again does not consider zoom control of the beam; U.S. Pat. No. 4,063,819 to Hayes which likewise describes only a scanning method; U.S. Pat. No. 4,100,404 to Johnson et al. in which a pseudo-zoom function is implemented in a beam projector by varying the size of the projected image and the size of the illumination spot; U.S. Pat. No. 4,161,652 to Moreau et al. which also shows only a scanning method; U.S. Pat. No. 4,168,908 to Cubalchini which describes a precision pointing and tracking system, but again does not deal with a zoom function; U.S. Pat. No. 4,209,244 to Stewart, Jr. which describes a way of scanning a pattern in orthogonal directions with a single galvanometer, the zoom function being implemented by having several laser sources of different sizes; U.S. Pat. No. 4,446,363 to Lakin et al. in which a projection system is described but no zoom function is considered; U.S. Pat. No. 4,559,445 to Hedin et al. which also describes a projection system that does not have a zoom function; and U.S. Pat. No. 4,647,761 to Cojan et al. which describes a means of steering the field of view of an optical system over wide angles but does not contain a zoom function.